The invention relates to a method for machining a blank by means of a tool for producing a finished part.
To machine a blank by means of a tool for producing a finished part, the tool is moved on predefined guide paths and paths, respectively, which may be calculated using software. Each single guide path comprises an arbitrary number of successive path segments and path sections, respectively.
Regarding the path segments, there are machining segments and connecting segments. Along a machining segment, the tool engages the material of the blank to perform material removal. A connecting segment leads from the end point of a machining segment to the start point of a subsequent machining segment. Along a connecting segment, usually the tool does not engage the material of the blank. This can be achieved by retracting the tool to some predefined clearance distance above the blank or the workpiece. In that case, usually a higher forward feed is used, e.g. the predefined “rapid” forward feed. However, along a connecting segment, the tool may as well engage the material of the blank and remove material thereof. The connecting movement then takes place directly within the machining area between two machining segments following to each other, e.g. when using a line-by-line machining pattern.
Generally, it is advantageous to perform connecting movements as efficient as possible in order to reduce total machining time. This is even more important for machines used in the production mode of a series production. However, reducing machine load and respecting the acceleration characteristics of the machine is just as important as efficiency. This is especially relevant in the context of high-speed machining, where extremely high forward feeds and strong accelerations are common. To clarify the technological background, in the following some essential technical aspects of machine tools will be explained in more detail.
Machine tools used for milling differ in manifold ways in terms of structure and way of operation. First of all, the number of controllable linear and rotational axes may be different. Three axis machines are able to control three linear axes (X, Y and Z) but no rotational axes (A/B and C). Accordingly, the tool is always oriented orthogonal to the machine table, in other words parallel to the Z axis of the Cartesian machine coordinate system. Four axis machines are additionally able to control one rotational axis (A/B) in order to tilt the tool relative to the Z axis of the machine coordinate system. Five axis machines are most capable in that also the second rotational axis (C) can be operated as well, preferably in such a way that a simultaneous control of all these five available axes is enabled. Such simultaneous control of all axes provides maximal freedom in guiding the tool. However, toolpaths to be calculated for the purpose five axis machines have to contain not only positional data for each machining point but also information with respect to angular values to control both the rotational axes.
A further distinctive feature of five axis machines is the kinematic behavior, i.e., the technical implementation of the control of rotational axes. For each of these axes, a given angle can be adjusted either in swiveling the tool head or, alternatively, in (inversely) swiveling the machine table. Depending on the machine manufacturer, there are many different possibilities: head-head kinematics (both rotational axes located in the tool head), table-table kinematics (both rotational axes located in the machine table) and numerous kinds of mixed kinematics. In this context, in particular the so-called dynamics of a machine is of importance. The dynamic properties determine the acceleration characteristics of the linear and rotational machine axes. There are great differences between the machines depending on the machine kinematics and drive technology as used. However, within a machine, i.e. between the individual axes, the dynamics may vary. For example, often the Z linear axis will have different acceleration properties than the X and Y linear axes. This is called anisotropic axis acceleration.
Machines for high-speed processing (high-speed cutting) represent yet another special category because of the specific requirements. They are characterized by extremely high spindle speeds and strong dynamics, i.e. very high forward feeds and strong accelerations of axes. This still further increases machine load effects resulting from unsuitable tool guidance strategies.
In known methods for machining a blank 110 by means of a tool 112 for producing a finished part according to the preamble of claim 1, as for example described in DE 696 24 093 T2 and/or shown in FIGS. 7a to 7c, the tool 112 is moved on a guide path 114 comprising at least three successive or subsequent path segments 116, 118, 120 in form of two machining segments 116, 120 and one connecting segment 118 which connects the two machining segments 116, 120 with each other. For example, the tool 112 may be a milling tool, a drilling tool or a laser tool.
FIGS. 7a to 7c show different application examples for the machining of holes 122, 122′.
In its simplest form, the connecting segment 118 has a linear shape, corresponding to FIG. 7a. The connecting movement of the tool 112 includes vertical retraction to some clearance distance above the blank 110. First, using an increased forward feed, the tool 112 is lifted vertically from an end point 124 of the first machining segment 116, which coincides with the start point of the connecting segment 118, to the first target point 126 at the defined clearance distance. Next, a linear horizontal connecting movement at this distance or height leads to the second target point 128. Finally, the tool 112 is lead and, respectively, moved down vertically from the second target point 128 to the start point 130 of the second machining segment 120, which coincides with the end point 130 of the connecting segment 118.
As shown in FIG. 7b, the connecting segment 118 again has linear shape. However, to avoid a collision between the blank 110 and the tool 112 in case of an obstacle during connection and the connecting movement, respectively, the clearance distance is enlarged.
Similar situations frequently arise in 5-axis machining, if the tool 112 is moved from one side 132 of the blank 110 to be machined to the next side 134 to be machined. In this case the critical portion of the blank has to be bypassed while considering the defined clearance distance, in the simplest case following a sequence of lines passing along a target point 136, such that the resulting connecting movement of the tool 112 has a polygonal shape as shown in FIG. 7c. 
FIGS. 7d and 7e show further application examples relating to the machining of surfaces 138 of the blank 110.
According to FIG. 7d, the connecting movement of the tool 112 in form of for example a milling tool may be carried out for surface machining of surfaces 138 between the two machining segments 116, 120 via the connecting segment 118.
Furthermore, as shown in FIG. 7e for instance, in the context of surface machining, connecting movements of a tool 112 in form of a milling tool may also occur within a local machining area 140, where machining segments are connected directly, i.e., without vertical retraction and thus possibly by engaging the material. Exemplary simple linear connecting movements are provided on the connecting segments 118, 118-1, 118-2 etc. between the machining segments 116, 120, 116-1, 120-1, 116-2, 120-2, 116-3 etc. of a line-by-line zigzag machining pattern with alternating advance direction.
With all of these methods the abrupt direction changes at the start and the end of the horizontal linear connecting movements, e.g. after the vertical retraction and before the vertical touch down, respectively, and at the target points 126, 128, 136 of the afore described linear and polygonal, respectively, shaped connecting segments 118, have been proved as most disadvantageous. Accordingly, such abrupt direction changes are unfavorable because they lead to high machine load, especially at high forward feeds.
For its improvement, in particular for a more machine gentle movement of the tool 112, other known methods therefore propose using curved connecting segments 118, as for example illustrated in FIGS. 8a to 8c for drilling process and in FIGS. 8d and 8e for surface machining.
In this context, in the embodiments of FIGS. 8a to 8e, which correspond to these ones of FIGS. 7a to 7e, curves are used that enable a smooth, i.e. tangent-continuous, course or run of the connecting movements of the tool 112. The transition between the connecting segment 118 and the foregoing and following machining segment 116, 120, respectively, thereby is (also often) tangent-continuous.
As shown in FIG. 8c, curved connecting segments 118 may be also used for a multilateral machining of the blank 110 from one side 132 to the other side 134, if the portion to be circumscribed or bypassed has to be taken into account when calculating the connecting segment 118 and curve, respectively.
Curved connecting segments 118 may also be applied within local machining areas 140, as shown in FIG. 8e. 
However, in practice all these methods have proven to be disadvantageous as well. The reason is that the calculation of the connecting segments 118 or connecting curves is based on geometric aspects only. The shape of a connecting segment or a connecting curve exclusively depends on the positions of the points to be connected and possibly on the local tangents adjacent thereto. A calculation of the connecting segments 118 or connecting curves does not respect any non-geometric aspects. In particular, forward feed relationships are not considered. This is very disadvantageous with respect to a machine load reducing and efficient process, also considering the acceleration characteristics of the machine. Additionally, in general the calculated connecting segments 118 or connecting curves have a much greater length than it is actually required, considering the given forward feed values. Moreover, such methods also do not consider the individual dynamic properties of a machine and a possibly anisotropic axis acceleration profile of a machine.